1. Field of the Invention
The present invention pertains to a marine riser system made up of a plurality of end to end connected tubular riser sections which are formed of tubular load bearing composite bodies formed of a filament wound fiber/resin matrix having an inner protective liner and an outer protective and buoyant shell structure. The configuration of the riser sections from the seabed to a surface connection varies to accommodate specific conditions at each elevation.
2. Background
The continued development of offshore hydrocarbon deposits has presented problems in the design of structures to provide access to and production of fluids from these deposits. One of the more vexatious structural engineering problems pertains to the provision of the marine riser system used in drilling and in the production of well fluids from submarine deposits. The development of hydrocarbon deposits in water depths exceeding 1500 feet is now contemplated and in process. The provision of a tubular conduit up to 24 inches in diameter or greater extending and supported over several thousand feet from the water surface to the seabed presents a unique engineering problem.
The provision of tubular steel riser sections in manageable lengths which will permit makeup and break out of a riser string and which will be adequate strength to prevent rupture, bending or collapse requires tubular wall thicknesses and supporting structure which become selflimiting. In order to reduce the strength and size requirements of tensioning and heave compensation apparatus for supporting the upper end of a riser string and to limit the buoyancy requirements of the vessel, it has been contemplated to provide various types of buoyancy structures which are disposed around steel and other metallic riser sections to offset the requirements for supporting the riser column under suitable tension to prevent collapse. These buoyancy structures, sometimes in the form of large sleeves of buoyant material or air filled caisson type structures, are particularly cumbersome to deal with in making up or breaking down a riser system. Moreover, the monitoring of air filled caisson structures and the prospect of failure of these structures also makes them unattractive as means for supporting a submarine riser string.
In an effort to overcome the disadvantages of using steel, aluminum or other metals for submarine risers and similar tubular components it has been contemplated to provide nonmetallic composite structures such as the glass fiber-epoxy matrix composite pipe of the type described in U.S. Pat. No. 3,768,842 to A. G. Ahlstone. However, the provision of a glass fiber-resin matrix composite structure as the primary load bearing body suffers from a significant disadvantage. The modulus of elasticity of such structures is significantly less than steel, thereby requiring heave compensation or tensioning equipment on the floating platform or vessel to which the riser is attached which must undergo substantial linear excursion in order to maintain suitable tension in the riser string. Moreover, the elongation of a structure made up of a glass fiber reinforced plastic composite is particularly dangerous as regards the violent movement of the riser which would occur in the event of intentional disconnection of the riser or parting of the riser somewhere between the wellhead or other subsea anchor point and the heave compensating or tensioning equipment. This type of violent action cannot be risked around the expensive equipment and structure associated with floating drilling or production platforms and similar vessels.
Certain other considerations in the design of a marine riser also include the requirement that the riser be impermeable to fluids carried within the riser bore and the provision of suitable buoyancy and exterior wear and impact resistant structure which is exposed to the marine environment. Accordingly, the glass fiber reinforced resin composite structure suffers from certain disadvantages which make it unattractive for use as the main load bearing structure in a marine riser system. The desiderata of a structural load bearing member having a sufficient stiffness or high modulus of elasticity together with low density and properties of tensile strength which are comparable to or better than alloy steel or other high strength metals and suited for use as a marine riser type structure has eluded art workers prior to the development of the present invention.